The diamond attracts attention as a semiconductor light-emitting material since it has excellent potential semiconductor characteristic or optical characteristics in addition to mechanical, chemical and thermal characteristics. In particular, since it has negative electron affinity or very small electron affinity, an application to an electron source device that emits electrons from a surface thereof is expected. Further, it has a band gap of approximately 5.5 eV at room temperature, a possibility as a light-emitting element that emits light in an ultraviolet region or robust crystallinity, and hence an application to a high-power device is expected.
As an example of using the diamond as an electron source, a cold cathode using a boron-doped diamond is known. Further, field electron emission from a phosphorus-doped diamond has been also reported. There is also an example of thermionic electron emission from a nitrogen-doped diamond. The diamond is also utilized as an electron source using a PN junction, and the diamond is expected as a thermionic electron emission source at low temperature in particular. For instance, a Schottky diode of the diamond is known as an example of utilizing the diamond as a high-power element, an LED based on a PN junction of the diamond is known as an example of utilizing the diamond as a light-emitting element.
However, since the donor level formed by nitrogen is as deep as 1.7 eV in the nitrogen-doped diamond, resistance is higher than those of other semiconductors at low temperature in particular, and injection of charges, contact with an electrode or energization with a substrate is a serious problem. In particular, since a diamond substrate has a relatively high resistance, discontinuity emerges when Si or any other metal such as Mo having a lower resistance is utilized as a substrate because of a great difference in characteristics between the substrate material and the diamond, which is a cause of an increase in electrical resistance. Therefore, an electron emission amount is lowered in an electron source, current density is decreased in an electron device, and operating voltage is increased or light-emitting efficiency is reduced in a light-emitting device. In the phosphorus-doped diamond, the donor level formed by phosphorus is as small as 0.6 eV as compared with nitrogen, and electrons tend to flow at low temperature as compared with the nitrogen-doped diamond. Therefore, the phosphorus-doped diamond is the most promising thermionic electron emission source, but an example of observation of thermionic electron emission in a low electric field at low temperature has not been actually reported, and such emission has not been observed in experiments conducted by the present inventors.
In view of the above-described problem, it has been desired to provide an electron emission element that can obtain a high electron emission amount and a high current density even in a low electric field (e.g., 0.01 V/μm or below) at low temperature (e.g., 1000° C. or below) and to provide an electron emission apparatus using this electron emission element.